1. Field of the Invention
The present invention relates to a simulation system and method for simulating an artificial satellite (GPS satellite) used for a GPS (global positioning system).
2. Description of the Related Art
A conventional GPS simulator simulates GPS satellites by externally receiving GPS navigation data. The GPS navigation data is mainly obtainable from NASA (National Aeronautics and Space Administration), et cetera, through the Internet.
The GPS navigation data is mainly categorized into almanac and ephemeris data. The almanac data describes a parameter for figuring out approximate locations of all the GPS satellites and can be used for about two weeks. The time limit is governed by the orbits of the respective GPS satellites changing with time and is corresponding to the effective period of the navigation data.
Meanwhile, the ephemeris data is the one describing a detailed parameter of satellite orbital information about each satellite and is used for a GPS module (GPS receiver), etcetera, calculating the position of each satellite. The time limit of the ephemeris data is about two hours.
A common use of the GPS simulator is to utilize a method for synthesizing a navigation data by itself mainly from information of the almanac data to simulate a GPS radio wave (“GPS wave” hereinafter). It is also possible to operate a simulator by using the ephemeris data.
When evaluating the GPS module by using such a GPS simulator, the simulator is connected with the GPS module which receives a navigation data synthesized by the simulator and thereby calculates the current position. Positional information of a positioning point is inputted to the simulator in advance, a performance of the GPS module can be evaluated by comparing information about the positioning point outputted from the GPS module with the information about the positioning point inputted into the simulator.
The GPS simulator is capable of simulating many kinds of parameters such as the moving motion of a positioning point, an intensity of an emitted radio wave, and a multiple path, in addition to a parameter for comparing positions of the positioning point.
Recently there has been an increase in the number of mobile phones each equipped with a GPS module. Many of such GPS modules obtain navigation data from an alternative information source by way of the mobile phone communications line, rather than receiving it from the primary source, i.e., a GPS wave, thereby shortening the time for positioning itself (e.g., refer to a patent document 1).Patent Document 1Japanese patent application publication No. 2002-228739
There is also a system to figure out an approximate position of a mobile phone according to the positional information about a base station with which the mobile phone is communicating and to notify, by way of the communications line, the GPS module equipped in the mobile phone of the complement information about a GPS satellite. This system also contributes to shortening the time for positioning.
Such a positioning method is generally called a network assisted GPS system in which a GPS module equipped in a mobile phone and an assist server on a network side measure a position while the two synchronize with each other. A GPS module is also mounted on the assist server of course, obtaining GPS information from the GPS satellite continuously. Therefore, the GPS module equipped in the mobile phone is enabled to obtain a navigation data, et cetera, from the assist server by way of the network.
Moreover, there is another known method for using a signal generated by a GPS signal generator within a GPS simulator to synchronize a signal of another unit with the generated signal (e.g., refer to a patent document 2).
Patent Document 2
Japanese patent application publication No. 11-304900
In the meantime, a signal from the GPS satellite includes a pulse signal, once every one second, which is called a 1 PPS (pulse per second) signal. The signal is extremely accurate because it is generated by an atomic clock comprised by the GPS satellite. There is yet another known method for providing a precision clock time, in a clock time synchronizer apparatus using such a 1 PPS signal, by creating 1 PPS signals by two clocks having different clock times respectively, comparing these 1 PPS signals by a comparator and thereby feeding back the difference (e.g., refer to a patent document 3).
Patent Document 3
Japanese patent application publication No. 07-287083
A GPS module evaluation system by using the conventional GPS simulator is unable to measure a position accurately unless a GPS module mounted on a mobile phone operating by receiving a GPS wave emitted from the simulator synchronizes with a server which is receiving a GPS wave from a GPS satellite to be actually used. The conventional network assisted GPS system, however, has not provided a method for synchronizing the GPS module equipped with the mobile phone with the assist server accurately.